An X-ray CT apparatus is such that a subject is penetrated, an X-ray attenuated in accordance with an X-ray attenuation coefficient of a substance (composition) inside the penetrated subject is received in an X-ray detector, various kinds of process are carried out in response to an electrical signal obtained, and an X-ray CT image is obtained as a distribution diagram of the X-ray attenuation coefficient of the subject interior. The X-ray used is a polychromatic X-ray, and beam hardening correction in accordance with the substance is carried out.
Metal has high attenuation characteristics differing extremely from those of a substance contained in a normal subject, such as water or bone, and beam hardening has a large effect. Because of this, when metal such as an implant is included in an imaging range, this cannot be dealt with by beam hardening correction for water or bone, and an artifact appears in an image. An artifact caused by metal is a dark band accompanying beam hardening due to metal, or a streak artifact occurring because of a photon insufficiency.
These artifacts caused by metal are collectively called metal artifacts. These metal artifacts do not stop at a metal margin, but also affect a structure distanced from the metal, causing image quality to deteriorate, and causing visibility of a lesion to worsen.
An image reconstruction method that reduces this kind of metal artifact has been proposed (for example, refer to Patent Document 1 and Patent Document 2). The technology of Patent Document 1 is such that a metal portion in photographed projection data is identified by forward projection processing being carried out on a metal portion image extracted from a reconstructed image. Also, a value of peripheral composition or the like is substituted for the metal portion in the reconstructed image, a composition classification image is generated by applying an edge preserving smoothing process such as a bilateral filter to the value, and composition classification projection data are obtained by forward projection processing being carried out on the composition classification image. The metal portion of the photographed projection data is substituted in the composition classification projection data, post-correction projection data wherein the metal artifact has been corrected are obtained, these are subjected to image reconstruction, and the metal portion is added to the reconstructed image obtained, whereby a corrected image is obtained. Also, the technology of Patent Document 2 is such that a metal portion in photographed projection data is identified by forward projection processing being carried out on a metal portion image extracted from a reconstructed image in the same way as in Patent Document 1.
Also, a composition classification image is generated from the reconstructed image by substituting a predetermined value for each composition such as soft tissue, bone, and air, and composition classification projection data are obtained by forward projection processing being carried out on the composition classification image. The composition classification projection data are combined with the photographed projection data in which the metal portion has been identified, post-correction projection data wherein the metal artifact has been corrected are obtained, these are subjected to image reconstruction, and the metal portion is added to the reconstructed image obtained, whereby a corrected image is obtained. When combining, standardized projection data are obtained by a standardization (division) process being carried out on the photographed projection data with the composition classification projection data, and after data of a region corresponding to the metal are linearly interpolated with peripheral data in the standardized projection data, a reverse standardization process (multiplying by the composition classification projection data) is carried out, and image reconstruction is carried out, whereby a corrected image is obtained.
Also, the technologies of Patent Documents 1 and 2 are such that after data of a region corresponding to the metal are linearly interpolated with peripheral data in the photographed projection data, the image is reconstructed, whereby a corrected image is obtained.